1. Field of the Invention
The present invention relates to a ferroelectric memory including a ROM area which is composed of ferroelectric memory cells, and a method of reading data in the ferroelectric memory.
2. Description of the Related Art
A ferroelectric memory composed of memory cells each including a ferroelectric capacitor is developed as a semiconductor memory device which has both advantages of DRAMs and flash memories and EEPROMs. The ferroelectric memory operates the ferroelectric capacitor as a variable capacitor. The ferroelectric memory can hold data without supplied with voltage, by utilizing residual dielectric polarization which is left even when no voltage is applied to the ferroelectric capacitor.
The supply voltage (operating voltage) of the ferroelectric memory has been decreasing due to finer transistor structure, reduction in power consumption and the like. This often causes deterioration in data write characteristic to the ferroelectric capacitor and reduction in a read margin. Specifically, a hysteresis loop indicating the data write characteristic is smaller in size as write voltage decreases, resulting in lessening an amount of electric charge generated from variations in polarization charge during read operation.
Recently, there has been developed an IC card with security function incorporating the ferroelectric memory. The ferroelectric memory mounted on this type of the IC card has a ROM area as a part of memory cell area. The ROM area stores therein key data used in Public Key Infrastructure (PKI). A manufacturer of the ferroelectric memory writes key data into the ROM area (disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-243761). Writing key data into the ROM area of the ferroelectric memory in the manufacturing process makes it possible to, for example, identify a user of the IC card with high reliability.
Generally, for mounting the ferroelectric memory on the IC card, a ferroelectric memory chip is soldered to a printed-wiring board of the IC card. In case of soldering with infrared reflow, the ferroelectric memory is temporarily subjected to heat of approximately 230° C. Since the hysteresis characteristic of the ferroelectric memory has temperature dependency, the higher the temperature of the ferroelectric capacitor becomes, the smaller the hysteresis loop becomes (the depolarization of the ferroelectric capacitor). Therefore, even when key data is written into the ROM area to generate substantial residual dielectric polarization, a residual dielectric a polarization value is reduced by a subsequent soldering processing (heat treatment), so that the read margin decreases. In other words, written data is lost. This problem becomes more serious as the operating voltage decreases (the write voltage decreases).